Atomic-scale visualization of single atom formation in metal–organic frameworks

Abstract

Recently, non-noble metal single atoms (SAs) have emerged as a groundbreaking class of materials, offering enhanced efficiency, reduced metal consumption, and widespread applicability. In the present study, zeolitic imidazolate framework-8 (ZIF-8) acts as a template for encapsulating Fe(acac)₃ precursors and transforms into porous nitrogen-doped carbon (PNC) upon pyrolysis, effectively capturing Fe SAs at nitrogen defect sites. The evolution of Fe SA formation within the porous ZIF-8 structure was investigated using atomic-scale in situ high-resolution scanning transmission electron microscopy (HR-STEM) from 325 °C to 400 °C. The formation of SAs increases with temperature, e.g. the densities of SAs are 2.04 × 10⁵ μm⁻² and 4.21 × 10⁵ μm⁻² at 400 °C for 3 min and 30 min, respectively. More importantly, the formation rates of SAs are rapid in the early stage and then slow down with prolonged pyrolysis time. The two-stage formation process may be governed by the atomization of Fe atoms from dispersed and clustered Fe(acac)₃ precursors, respectively. Theoretical calculations were performed to understand the formation mechanisms of the two stages. These insights, facilitated by atomic-scale in situ HR STEM observation, offer precise control over synthesis pathways, thereby advancing the design of tailored SA materials for catalytic applications and beyond.